With a growing interest in renewable energy including the use of solar power, there is an increasing demand for more efficient solar cells. Solar cells or photovoltaic (PV) cells are devices that convert solar or light energy into electricity by the photovoltaic effect, and solar cells are widely used in devices ranging from satellites and other applications, including portable consumer electronic devices that are remote from a conventional power source, aircraft and terrestrial vehicles. Although the term “solar cell device” may sometimes be used to refer to a device that captures energy from sunlight, the terms “solar cell device” and “photovoltaic device” are interchangeably used in the present application regardless of the light source.
Solar cells or photovoltaic devices (PV devices) convert sunlight directly into electricity and generally are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy frees electrons loose from their atoms, which allows the electrons to flow through the material to produce electricity. The process of converting light (i.e., photons) to electricity is called the photovoltaic (PV) effect. In practice, solar cells are typically combined into modules that hold numerous cells (e.g., up to 40 or more cells), and a set of these modules (e.g., up to 10 or more) are mounted in PV arrays or solar panels that can measure up to several meters or more per side, with each cell typically only being up to 100 to 150 square centimeters in size. These flat-plate PV arrays can typically be mounted at a fixed angle facing the Sun (e.g., south) or they may be mounted on a tracking device that follows the position of the Sun to allow them to better capture the Sun's light throughout the day. Solar cells may also be formed using thin film technologies to use layers of semiconductor materials that are only a few micrometers thick.
Currently, III-V compound based photovoltaic devices are epitaxially grown on substrates and remain affixed thereto throughout fabrication and deployment as a solar cell. In many cases, the substrates can be approximately 150 μm thick. Having substrates with such thickness may introduce a number of undesirable consequences for a solar cell.
One such undesirable consequence is weight. The thick substrate can make up a large percentage of the overall weight of the resulting solar cell. In certain applications, such as space applications, weight and size of a solar cell can be significant given the liftoff capability of the selected launch vehicle.
Another undesirable consequence is poor thermal conductivity. The substrate increases the thermal impedance between the solar cell and a heat sink on which the substrate and solar cell may be mounted. The increased thermal impedance results in higher junction temperatures in the solar cell, which, in turn, reduces the efficiency of the solar cell.
Another undesirable consequence is environmental impact. The substrate serves no purpose other than as a mechanical support for the solar cell. In addition, to achieve a substrate thickness of approximately 150 μm, it is typically necessary to remove, mechanically or chemically, part of the substrate, which amounts to further waste.
Another undesirable consequence is lack of flexibility. A 150 μm thick substrate is rigid, which means that the solar cell cannot be mounted on a curved surface and cannot be rolled up for easy storage, thus limiting their potential applications.
Although, it is desirable that the substrate of a solar cell be as thin as possible to reduce the weight and to increase the thermal conductivity, thin substrates can also present undesirable difficulties. If the substrate is too thin, the III-V compound solar cell can become so fragile that it is very difficult to handle. For example, a layer of Gallium Arsenide (GaAs) that is 100 mm in diameter, but only 2-10 μm thick, tends to crack and break when subjected to even very gentle handling. This consideration applies particularly to whole wafers of III-V compound solar cells. That is, the thinner the substrate, the more difficult it becomes to fabricate whole wafers of III-V compound solar cells without a decrease in yield due to breakage and handling damage.
Accordingly, a thin film III-V compound solar cell and methodologies for fabrication of thin film III-V compound solar cells that are highly-efficient, flexible, and formed as sheets (such as solar sheets) are highly desirable.
Solar sheets offer a convenient and effective method of generating electrical power for space, airborne, and terrestrial applications. The problem, however, with conventional solar sheets is that they are typically heavy, inefficient, and bulky. Moreover, conventional solar sheets have a low specific power (power generating capacity per unit mass), low areal power (power generating capacity per unit area), and high areal mass density (mass per unit area).